1. Field of the Invention
The present invention relates to an impedance stabilizing unit and a high-frequency module using the unit. More particularly, the present invention relates to an impedance stabilizing unit and a high-frequency module using the unit to be used in the transmission side of a radio equipment.
2. Description of the Related Art
In FIG. 8, a RF block diagram of a portable telephone terminal as a general radio equipment is shown. The portable telephone terminal operating in a single frequency band such as 800 MHz, 1.5 GHz, etc. includes an antenna ANT, a duplexer for antenna DPX, a transmission side radio signal portion Tx, a reception side radio signal portion Rx, and a synthesizer portion SYN. The input of the transmission side radio signal portion Tx is connected to a modulator portion MOD and the output of the reception side radio signal portion Rx is connected to a demodulator portion DE respectively. The transmission side radio signal portion Tx is composed of a high output amplifier PA, a band-pass filter F1, a low-pass filter F2, a mixer MIX1, an isolator ISO, and a directional coupler COU and performs a function to output a transmitting signal in receipt from the modulator MOD to the antenna ANT through the duplexer for antenna DPX. On the other hand, the reception side radio signal portion Rx is composed of a low-noise amplifier LNA, band-pass filters F3, F4, and a mixer MIX2, and carries out a function to output a receiving signal received from the antenna ANT to the demodulator DE through the duplexer for antenna DPX.
Generally, the impedance on the side of an antenna ANT and the impedance on the side of a high output amplifier PA are set to be 50 ohms (.OMEGA.) respectively and are made nearly the same, but there are cases that the impedance on the side of the antenna ANT is gradually increased while in use. In those cases, the impedance on the side of the antenna ANT is nearly matched to the impedance on the side of the high output amplifier
with a current made to flow through the high output amplifier PA. However, if the impedance on the side of the antenna ANT becomes too much different, for example, by breaking of the antenna ANT, the high output amplifier PA breaks down by an excessive current flowing the high output amplifier PA. In order to prevent such a phenomenon and stabilize the impedance on the side of the antenna ANT, an isolator ISO is arranged between the antenna ANT and the high output amplifier PA in the same way as the construction of FIG. 8.
However, in the above-mentioned portable telephone terminal as a radio equipment, there was a problem of having the insertion loss on the transmission side increased, because an isolator ISO is arranged between the antenna ANT and the high output amplifier PA in order to stabilize the impedance on the antenna side.
Further, there was also a problem of being difficult to make the transmission side radio signal portion small-sized, light-weighted, and low-cost.